After leaving my former employer, I was asked to come back, and was
offered a $10000 signing bonus as incentive to accept. Terms of the
offer were contingent upon the successful completion of a background
check.

I filled out a form for the background check (which was 100%
accurate). The company, however, did not use the information I
provided, but used old info from  my original application (circa
2002), which was not 100% accurate but passed the background check in
2002. I also provided a new application with the corrected info, so I
was forthcoming from the start of this negotiation -- thus, they still
made the offer w/bonus.

Essentially, the background check using the OLD info was not
successful (obviously). The company rescinded the offer one business
day before I was to start. The company subsequently sent me a letter
stating it was because of erroneous information on my [NEW]
application that the offer was rescinded, which as noted was not the
case.

Employment is at-will in that employer's state (PA), so I guess I can
understand if an offer is rescinded. Are there any grounds, however,
to get the bonus? Even punitive in that the company did not follow its
own policies for hiring? Thanks.

-John

It can't hurt to consider a small claims action. Pennsylvania is one
of the more liberal small-claims states with a ten thousand dollar
limit. Talking to a lawyer may save you the time and energy of suing,
or may cost you more than the filing fee. Between filing, service,
orders, etc, you can sue in small claims court in PA for under about
$200, max, and very likely less.

Now as to whether you actually have a case, you need to talk to an attorney.